The Principles of Geology
There are several basic principles that geologists use to figure out the history of a rock:
- Uniformitarianism
- Original horizontality
- Superposition
- Cross-cutting relationships
- Walther’s Law
Uniformitarianism
The principle of uniformitarianism states that processes that alter the earth’s crust are the same processes that occurred millions of years ago. Furthermore, the results of processes today are the same as the results of the same processes millions of years ago. This means that we can take our observations of processes that occur today, and observations of the results and know the process that formed it when we see that same result in the rock record. For example, you can look in a stream and see ripple marks in the sand, formed by the flow of water over the sand. If you see ripple marks in the rock record, you can know that a similar process was at work.
Original Horizontality
The principle of original horizontality states that sediment is deposited horizontally. This is sometimes easier to envision with liquids: imagine pouring water into a cup. The surface of the water is perfectly flat - horizontal. If you dump that water into a bowl, the surface remains flat. Now imagine that you have a jello mixture in the bowl - if you chill it and it solidifies, and then pour a different color on top, You have the two flat layers of jello, one on top of the other. This is similar to how sedimentary rocks form. As water moves sediment from high regions, like mountains, to low regions, like the ocean, the energy of the system decreases until the sediments are deposited in a basin, like a lake or an ocean. More sediment is deposited on top, and over time the whole sequence lithifies (sort of like the jello did in the fridge). The rocks remain horizontal until a force acts on them, pushing (or pulling) them out of their original orientation.
Video: Laws of original horizontality (1:22)
Dr. Ben Kotrc: I'm crouched amongst these beautiful banded rocks known as the marble bar. And one thing that might be the first thing for you to notice, is that these bands are not horizontal, but they're inclined at quite a steep angle. And you can see that all of the bands, all around in the background, are inclined at that same steep angle. One of the things we'd really like to know is whether these rocks were originally sediments. And one feature of sedimentary rocks that we know from observing rocks and the processes that form rocks around us, is that sediments tend to form horizontally in flat sheets. So for example, that's because of gravity, so if for example, if I pick up a handful of sand and drop it on the ground, it's going to come to rest horizontally, not at an angle. So one thing we want to look for is evidence that these rocks were deposited horizontally originally. And if so, we'd like to know which direction is up. So whether these rocks originally faced up this way or faced up that way. So if these rocks were originally horizontal, they could have come to lay like this in one of two ways. They could either have tilted like this, or they could have tilted over like this.
Superposition
This principle states that a sequence of rocks in their original orientation will have the oldest rock on the bottom and the youngest rock on the top. A simple way to think about this is that for something to be on top of something else, for example in order to put a book on top of a table, the table has to be there. If the table isn’t already there and you put the book down, it falls to the floor (and note! The floor had to be there for the book to land on it.). The same is true of rocks. In order to deposit a sandstone on top of a limestone, the limestone has to already be there. Knowing this, geologists can figure out the relative ages of rocks on top of each other. This image of the Grand Canyon is a good example with the older rocks on the bottom and the rocks are younger as you move up the cliff face, with the youngest rocks on top.
Cross-Cutting Relationships
Similarly to the principle of superposition, a rock must already be in place to be cut by a fault, igneous intrusion or erosion. By carefully examining which rock units are cut by faults or intrusions, or which rock units have been weathered, geologists can further determine the relative ages of rocks and features within them. As shown in the picture below the horizontal, brown rock layers in the Grand Canyon were deposited over time with the oldest rocks on the bottom. Subsequently, hot magma was injected up through the brown sedimentary rock layers as seen with the diagonally-emplaced darker igneous rock layer. The fact that the dark layer seems to cut so cleanly through the older to younger sedimentary layers is evidence that it came along after the youngest rock layer the igneous intrusion cut through (It is not possible to cut layers before they exist!)
Walther's Law
Walther’s law is a little different from the previously discussed geologic principles, but it is just as important. Instead of dealing only with relative time, Walther’s law deals with relative space through time. Walther’s law states that depositional environments that are laterally adjacent on the surface of the earth will also appear in succession in a stratigraphic sequence. If there is rock missing, there is missing time in the rock record which is known as an unconformity.
Okay, that was a lot to take in. Let’s break it down, starting with laterally adjacent and depositional environments. If two things are right next to each other, they are laterally adjacent. A depositional environment is simply a place sediment can be deposited. Different types of sedimentary rock form in different depositional environments, so geologists can often figure out what existed at a particular place millions of years ago. Some examples of depositional environments include a meandering river, delta, beach, lake, swamp, shallow marine, and deep marine settings. So what are laterally adjacent depositional environments? Two depositional environments are considered laterally adjacent if you can walk from one to the next without anything in between. (If there IS something in between, that something is the laterally adjacent environment!) Imagine that your kitchen has a door to an outside porch, and your porch is just one step above your grassy backyard. You can walk from the kitchen to the porch without encountering the grassy backyard. That means your kitchen and the porch are laterally adjacent. Can you get from your kitchen to the grassy backyard without encountering something else? Nope, you have to cross the porch to get the yard. The kitchen and the yard are not laterally adjacent. The image below shows typically laterally adjacent depositional environments.
Changes in depositional environments are driven by changes in base level, or the elevation of the terminal body of water (often, but not always sea level!). When base level changes, the depositional environments shift to achieve a new equilibrium. If sea level falls, you might shift the depositional environment image to the right. However, you can’t move the mountains, so you end up stretching out the environments in between the mountains and the ocean. Where you once had a delta, now you might have a meandering river. If instead, you raise sea level, you would move the ocean to the left, squishing the environments in between. The result could be a delta where you once had a meandering river. Think of the beach as the main focus: If the beach moves towards where the ocean used to be, sea level has fallen and your sediments prograde. If the beach moves away from where the ocean used to be, sea level has risen and your sediments regress.